The examination of a stamp or cover by an acknowledged expert to determine if it is genuine. As standard procedure, an expert or expertizing body issues a signed certificate, often with an attached photograph, attesting to the item's status.
the process by which an expertizing service renders an authoritative opinion on the genuineness of stamps and covers. Most authoritative opinions are provided by the American Philatelic Expertizing Service (a service of the APS), and by The Philatelic Foundation.